1. Field of the Invention
The present invention relates in general to optical focusing devices, and it particularly relates to a high numerical aperture (NA) optical focusing device. More specifically, the present invention relates to an optical focusing device having a conically shaped incident facet, for use in data storage systems such as optical and magneto-optical (MO) disk drives.
2. Description of Related Art
In a MO storage system, a thin film read/write head includes an optical assembly for directing and focusing an optical beam, such as a laser beam, and an electromagnetic coil that generates a magnetic field for defining the magnetic domains in a spinning data storage medium or disk. The head is secured to a rotary actuator magnet and a voice coil assembly by a suspension and an actuator arm positioned over a surface of the disk. In operation, a lift force is generated by the aerodynamic interaction between the head and the disk. The lift force is opposed by equal and opposite spring forces applied by the suspension such that a predetermined flying height is maintained over a full radial stroke of the rotary actuator assembly above the surface of the disk.
A significant concern with the design of the MO head is to increase the recording or areal density of the disk. One attempt to achieve objective has been to reduce the spot size of the light beam on the disk. The diameter of the spot size is generally inversely proportional to the numerical aperture (NA) of an objective lens forming part of the optical assembly, and proportional to the wavelength of the optical beam. As a result, the objective lens is selected to have a large NA. However, the NA in objective lenses can be 1 if the focusing spot were in air, thus limiting the spot size.
Another attempt to reduce the spot size and to increase the recording areal density has been to use solid immersion lenses (SILs) with near field recording, as exemplified by the following references:
U.S. Pat. No. 5,125,750, titled xe2x80x9cOptical Recording System Employing a Solid Immersion Lensxe2x80x9d.
U.S. Pat. No. 5,497,359, titled xe2x80x9cOptical Disk Data Storage System With Radiation-Transparent Air-Bearing Sliderxe2x80x9d.
Yet another attempt at improving the recording head performance proposes the use of near-field optics, as illustrated by the following reference:
U.S. Pat. No. 5,689,480, titled xe2x80x9cMagneto-Optic Recording System Employing Near Field Opticsxe2x80x9d.
A catadioptric SIL system is described in the following references, and employs the SIL concept as part of the near-field optics:
Lee, C. W., et al., xe2x80x9cFeasibility Study on Near Field Optical Memory Using A Catadioptric Optical Systemxe2x80x9d, Optical Data Storage, Technical Digest Series, Volume 8, pages 137-139, May 10-13, 1998; and
xe2x80x9cParallel Processingxe2x80x9d, 42 Optics and Photonics News, pages 42-45, June 1998. While this catadioptric SIL system can present certain advantages over conventional SILs, it does not appear to capture the entire incident, collimated beam. This represents a waste of valuable energy that could otherwise increase the evanescent optical field.
Other concerns related to the manufacture of MO heads are the extreme difficulty and high costs associated with the mass production of these heads, particularly where optical and electromagnetic components are assembled to a slider body, and aligned for optimal performance.
One aspect of the present invention is to satisfy the long felt, and still unsatisfied need for a near-field optical or MO data storage system that uses an optical focusing device that has combines a conically (or axicon) shaped incident facet with a peripheral reflector or reflecting surface. According to one design, the peripheral reflector can have a shifted parabola shape. This optical focusing device captures substantially the entire incident beam and the peripheral reflector focuses it at a focal point with extremely small aberrations, thus improving the overall efficiency and performance of the data storage system.
Another aspect of the present invention is to provide an optical focusing device with relatively high manufacturing tolerance values, due largely to its simple structure and insensitivity to axial displacement of the top and bottom surfaces.
Yet another aspect of the present invention is to provide an optical focusing device with a relatively high NA by controlling the conic constant or other coefficients of the incident facet, and the peripheral facet which substantially compensate for the conical factors and the aberrations introduced by each other.
Still another aspect of the present invention is to provide an optical focusing device with quasi-flat facet using diffractive optical elements or Fresnel optics, thus making the mass production fabrication possible.
A further aspect of the present invention is to provide an optical focusing device that adds focusing power to the incident facet, by controlling certain parameters such as the angle or curvature of the incident facet.
The optical focusing device includes an incident central facet upon which an optical beam impinges, and a high-index glass body through which the incident optical beam passes toward a bottom reflective surface. The bottom reflective surface is substantially flat, and reflects the optical beam through the body, toward a peripheral reflector. The peripheral reflector focuses the optical beam toward a focal plane on which the focal spot is formed. The focal plane is defined within a pedestal that forms part of the optical focusing device, and that extends from the bottom reflective surface. The central facet is conically shaped for refracting the incident optical beam away from the pedestal, onto the bottom reflective surface. The peripheral reflector surrounds the central facet and can have various appropriate shapes, such as an aspherical shape, or a tilted parabolic shape, which compensates for the conical factor and aberrations introduced by the central facet. The top surface includes the central facet and the peripheral facet, and can be made substantially flat using diffractive optical elements or Fresnel optical elements.
In a data writing or reading mode, the incident optical beam, such as a laser beam impinges upon the central facet and is refracted or diffracted thereby. The laser beam can be collimated, convergent or divergent, and it passes through the transparent body, and impinges upon the bottom reflective surface. The laser beam is then reflected by the bottom reflective surface, through the body, onto the peripheral reflector. The laser beam is then either reflected, reflected and refracted, or reflected and diffracted by the peripheral reflector to form a focused beam at the focal point. The focal point is preferably located at, or in close proximity to a pedestal edge, along a central axis, in very close proximity to the disk. This will allow the focused optical beam to propagate toward, or penetrate the disk through evanescent wave coupling, for enabling the transduction of data to and from the disk.